The purpose of this study is to test the safety and determine the optimal dose of a new experimental drug, OMP-21M18, when given in combination with FOLFIRI, a chemotherapy regimen consisting of the following three medications: folinic acid (leucovorin), 5fluorouracil and irinotecan. The administration of these three medications is a standard treatment for advanced colorectal cancer. OMP-21M18 is a humanized monoclonal antibody (a protein made in the laboratory) developed to target cancer stem cells. The way the body handles OMP21M18 will also be investigated.